The present invention relates to magnetic memory cells used to store digital electronic data, and more particularly to an improved magnetic memory cell which functions as a random access memory.
The dipole magnetic moments of neighboring atoms within a small region, or domain, of a thin film of magnetic material align themselves when placed in a sufficiently strong external magnetic field. This alignment of magnetic dipole moments is unique to magnetic materials (Fe, Co, Ni, Gd and Dy) and takes place despite the random motion generally undergone by atoms within any material. The material orientation of the magnetic dipole moments remains after the external magnetic field is removed.
Transition regions exist between any two domains which do not have the same alignment of magnetic dipoles. The transition regions between such domains are called domain walls. Different types of domain walls typically exist in magnetic material, each unique as to the orientation of the magnetic field existing within or comprising the domain wall. Within the type of domain wall referred to as a Neel wall, the magnetic field rotates in the plane of the film 180.degree., thus separating two antiparallel domains. Reversing the magnetic field direction in a small portion of a Neel wall results in the creation of a different type of domain wall, the cross tie. The cross tie magnetization constitutes a separate stable magnetic domain.
The characteristic magnetic fields of the domain wall types remains unchanged in the absence of an external magnetic field of a predetermined strength. In the presence of an external field of the predetermined strength, however, the magnetic state of a domain wall at any given location can be changed.
The stable magnetic domain states of the magnetic film represented by the domain wall magnetization fields may be utilized within a memory system for the storage of digital data. U.S. Pat. No. 3,868,659, issued on Feb. 25, 1975 to Leonard J. Schwee, discusses the use of thin film magnetic materials as data storage devices. A more recent disclosure of such use of thin film magnetic materials is contained in U.S. Pat. No. 4,246,647, which on Jan. 20, 1981 to Johnson et al. In both of those patents, the memory disclosed is a serial memory, i.e., once a data bit is entered at one end of the memory, it is passed through the memory and cannot be removed until all data entered ahead of it has been removed. The operation of such a memory is fully described in the above-referenced patents. Such a serial memory has obvious limitations, in that it is often desirable to randomly access data which has been stored in the memory. In a serial access memory, to access a given data bit, it is necessary to first read out all data which was entered before the data bit of interest.
Long serial access type magnetic memory systems, such as the ones described above, are subject to drawbacks in that the magnetic fields within the thin film strips are easily disturbed by the ambient external magnetic fields which exist in the area surrounding the magnetic memory system, thus disturbing the location of the Neel walls or the cross-ties and the data these features represent. The lengthy Neel walls which exist in such serial memories are particularly easy to disturb, and have a tendency to move from the center of the magnetic strip to one side on the other of the strip. As a result of these disadvantages, there are many opportunities for error in reading and writing into such a memory.